Harmful algal blooms (HABs) of A. fundyense, more commonly known as "red tides", are a serious economic and public health concern in the Gulf of Maine. Until recently, there was very little known about the mechanisms regulating the observed spatial and temporal distributions of A. fundyense in this region. In the beginning of this work a review of previous research on A. fundyense and the mechanisms controlling their spatial and temporal distributions in the Gulf of Maine is presented. One of the major conclusions that can be drawn from previous work is that a thorough understanding of the interactions between river plumes is essential to our understanding of this problem. The rest of this thesis intends to contribute to the understanding of these plume interactions and their effect on the transport of A. fundyense. Mixing between two interacting river plumes with various buoyancies is investigated through laboratory experiments. These experiments indicate that under these idealized conditions, there was little mixing between the plumes after their initial interaction. A numerical model is used to explore the effects of river mouth size and flux variations on the interaction between two plumes. It is shown that based on river mouth geometry and flow rates the effect of the southern plume on the path of the northern plume can be predicted. In the final section a simple NP model is coupled with the physical model to explore the possible effects of river plume interaction on the distribution of A. fundyense. Based on our modeled results, it appears as if the southern river under certain conditions could temporarily act as a shield preventing A. fundyense from reaching the coast but that this was not a permanent state.